TWI489081B - Low coherence interferometry using encoder systems - Google Patents

Description

Low coherence interference technique using an encoder system

The present invention generally relates to a low coherence interference technique using an encoder system.

Optical encoders measure distance and shift by optically reading scale gauges move. Unlike the distance measuring interferometer (DMI), the ruler scale defines the basic length unit, not the wavelength of light. The interferometer (encoder read head) used to read the ruler is typically quite close to the ruler to minimize perturbations. The read head directs light to the ruler and restores one or more diffraction steps to determine movement along the ruler plane. The close proximity of the read head to the ruler causes undesired diffraction steps to be intercepted by the read head, resulting in measurement errors. For example, a 2D ruler produces a diffraction order along four directions. When using the same dimming (eg, laser light), reflections from these additional beams or ghost beams that are reflected off other optical interfaces interfere with the measuring beam and cause measurement errors. While the geometry of the encoder system can be configured to block some of these unwanted beams, it is difficult to anticipate all ghost beams, especially if the grating or readhead is in dynamic motion due to multiple reflections. The ghost beam will still cause measurable errors, and the platform movement will dynamically change the direction of the ghost beam.

The technical features of the present disclosure relate to low coherence interference techniques using an encoder system. The encoder system can be used to minimize or eliminate unwanted unwanted ghost beams through the use of low coherent illumination and coupling chamber structures. The encoder system includes a low coherent source and two interferometer chambers coupled in series. A coupled chamber encodes heterodyne modulation and defines a system optical path difference (OPD). The other chamber contains a readhead interferometer. This combination is particularly useful for encoders because the range of motion perpendicular to the plane of the encoder ruler is limited. By including the source coherence, this class is included The ghosts whose optical paths exceed this range no longer interfere with the test beam and are electrically excluded.

Various aspects of the invention are summarized below.

Generally, in a first aspect, the present disclosure features a method for determining information regarding a change in position of an encoder ruler, wherein the method includes dividing a low coherent beam into a first interferometer chamber a first light beam traveling along a first path of the first interferometer chamber and a second light beam traveling along a second path of the first interferometer chamber; the first light beam and the first light beam The two beams combine to form a first output beam; the first output beam is split into a measurement beam traveling along one of the measurement paths of the second interferometer chamber in a second interferometer chamber and along the a reference beam traveling along one of the interferometer chambers; combining the measuring beam with the reference beam to form a second output beam; detecting an interference signal based on the second output beam; and based on the interference from the interference The phase information of the signal determines the information about the change in position of the encoder ruler.

Implementation of such methods may include features of one or more of the following features and/or other aspects. For example, the methods can include adjusting an optical path difference (OPD) associated with the second interferometer chamber. Adjusting the OPD associated with the second interferometer chamber can include setting the OPD associated with the second interferometer chamber to be approximately equal to an OPD associated with the first interferometer chamber. A difference between the OPD associated with the second interferometer chamber and the OPD associated with the first interferometer chamber may be less than or equal to a coherent length of the low coherent beam. Adjusting the OPD associated with the second interferometer chamber can include adjusting an optical path length of at least one of the measurement path or the reference path Degree (OPL, optical path length). Each of the OPD associated with the first chamber and the OPD associated with the second chamber may be greater than a coherent length of the low coherent beam. In some embodiments, the OPD of the first chamber is equal to a difference between an optical path length (OPL) of the first path and an OPL of the second path, the OPL of the second path being different The OPL of the first path.

The methods can include directing the measurement beam toward the encoder ruler prior to combining the measurement beam with the reference beam, wherein the measurement beam is diffracted at least once from the encoder ruler. The methods can include moving a frequency of at least one of the first beam or the second beam in the first interferometer chamber. The second output beam may include a heterodyne frequency, the heterodyne frequency being equal to the frequency of the first beam and the second after moving the frequency of at least one of the first beam or the second beam A difference between the frequencies of the beams.

Generally, in another aspect, the invention features an interferometric system including a low coherent illumination source; a first interferometer chamber coupled to the low coherent illumination source to receive one of the illumination sources Outputting, the first interferometer chamber is associated with a first optical path difference (OPD); and a second interferometer chamber coupled to the first interferometer chamber to receive the first interferometer chamber An output of the second interferometer chamber is associated with a second OPD.

Embodiments of the interferometric system may include features of one or more of the following features and/or other aspects. For example, the first OPD can be fixed. In some embodiments, the second OPD is adjustable.

A difference between the first OPD and the second OPD may be less than the low One of the outputs of one of the illumination sources is the coherence length (CL). Each of the first OPD and the second OPD is greater than a coherence length (CL) of the output of the illumination source. The first OPD can be approximately equal to the second OPD.

The first chamber may include a first segment having a first optical path length (OPL) and a second segment having a second different OPL, the OPD of the first chamber being equal to the first OPL and the This difference between the second OPL.

The first chamber can include a frequency shifting device in the first segment, the frequency shifting device configured to move a frequency of light in the first segment during operation of the interferometric system. The frequency shifting device can include an acousto-optic modulator or a photoelectric phase modulator.

The second chamber includes a first segment having a first optical path length (OPL) and a second segment having a second OPL, the OPD of the second chamber being equal to the first OPL and the second A difference between OPL. At least one of the first OPL and the second OPL is adjustable. The first segment may correspond to a measurement path and the second segment corresponds to a reference path. The second chamber can include a diffractive encoder ruler, each of the first OPL and the second OPL being defined relative to a position of the encoder ruler.

The interferometric system can include a photodetector and an electronic processor configured to obtain heterodyne phase information from a signal detected by the photodetector during operation of the interferometric system . The second chamber can include a diffractive encoder ruler, and the electronic processor can be configured to obtain a degree of freedom with respect to one of the encoder gauges based on the heterodyne phase information during operation of the interferometric system Location information.

Certain implementations may have particular advantages. For example, in some implementations, The use of an interferometric system with low coherent illumination and a coupling chamber structure can aid in the elimination of unwanted ghost beams. A coupling chamber (the heterodyne chamber) can encode a heterodyne modulation and define a system optical path difference (OPD), while another chamber (the test chamber) can include a readhead interferometer . This combination is particularly useful for encoder interferometric systems where the range of motion of the encoder interferometric system perpendicular to the plane of an encoder ruler is limited. By selecting this range of the illumination source to include this range, the ghost beam of the optical path beyond this range does not interfere with the test beam and can be electrically excluded. In addition, the readhead interferometer can include a variety of different optical geometries as long as the chamber OPD limits are met. Moreover, the heterodyne chamber need not be positioned directly adjacent to the test chamber. Instead, the heterodyne chamber can be positioned away from the test chamber. The heterodyne chamber may be a source of superheat that may negatively affect the optical path length of the test chamber (eg, by causing a change in refractive index of the optical components within the test chamber), and thus causing a position The calculated error. By placing the heterodyne chamber at a location remote from the test chamber, errors due to overheating from the modulator chamber can be avoided in some implementations.

The details of one or more embodiments are set forth in the drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims.

This disclosure relates to low coherence interference techniques using encoder systems. under The face disclosure system is arranged in three paragraphs. The first paragraph of the disclosure is entitled "Interferometric Optical Encoder System", which is a general description of how an interferometric optical encoder system can operate. The second paragraph of the disclosure is entitled "Low Coherent Optical Encoder System" for example optical encoder systems and their operation in accordance with low coherent illumination and coupling chamber structures. The third paragraph of the disclosure is entitled "Micro-Shadow Tool Application", which relates to aligning an optical encoder system into a lithography system.

Interferometric optical encoder system

Referring to FIG. 1 , the interferometric optical encoder system 100 includes a light source module 120 (including, for example, a laser), an optical device 110 , a measuring object 101 , a detector module 130 (eg, including a polarizer and a detector), and Electronic processor 150. Generally, the light source module 120 includes a light source, and may also include other components, such as a beam shaping optical component (such as a light sighting optical component), a lightguide component (such as a fiber optic waveguide), and/or a polarization management optical component (such as a polarizer and / or wave plate). Various embodiments of optical device 110 are described below. Optical devices are also known as "encoding heads." The Cartesian coordinate system is shown as a reference. In the example of Figure 1, the Y-axis extends in a direction perpendicular to the page.

The measuring object 101 is positioned along the Z-axis at a nominal distance from the optical device 110. In many applications, such as where an encoder system is used to monitor the position of a wafer platform or reticle stage in a lithography tool, the measurement object 101 is moved relative to the optical device in the X and/or Y direction while being relative to the Z The shaft maintains a nominal fixed distance away from the optical device. This fixed distance can be quite small (eg, a few centimeters or less). However, in such applications, the measurement The position of the body will typically differ by a small amount from the nominal fixed distance, and the relative orientation of the measuring object may also vary by a small amount within the Cartesian coordinate system. During operation, encoder system 100 monitors one or more of these degrees of freedom of measurement object 101 relative to optical device 110, including measuring the position of object 101 relative to the X-axis, and in some embodiments, measuring object 101 in some embodiments. A position oriented relative to the Y-axis and/or the Z-axis and/or relative to the angle of inclination and off-angle.

In order to monitor the position of the measuring object 101, the light source module 120 directs the input beam 122 to the optical device 110. The optical device 110 takes the measurement beam 112 from the input beam 122 and directs the measurement beam 112 to the measurement object 101. Optical device 110 also takes a reference beam (not shown) from input beam 122 and directs the reference beam along a path different from the measuring beam. For example, optical device 110 can include a beam splitter that splits input beam 122 into a measurement beam 112 and a reference beam. The measurement and reference beam may have orthogonal polarization (eg, orthogonal linear polarization).

The measuring object 101 comprises an encoder ruler 105, which is a measuring scale that diffracts the measuring beam from the encoding head into one or more diffracted orders. In general, encoder scales can include a variety of different diffraction structures, such as gratings or holographic diffraction structures. Examples of rasters include sinusoidal, rectangular, or sawtooth gratings. A grating is characterized by a periodic structure having a fixed pitch, but may also be a plurality of composite periodic structures (e.g., a graded grating). Typically, an encoder ruler can diffract a measurement beam into more than one plane. For example, the encoder ruler can be a two-dimensional grating that diffracts the measurement beam into diffraction orders in the X-Z and Y-Z planes. Encoder ruler A distance corresponding to the range of movement of the measuring object 101 is extended in the X-Y plane.

In the present embodiment, the encoder ruler 105 is a grating having a plurality of raster lines extending orthogonal to the plane of the page, parallel to the Y-axis of the Cartesian coordinate system shown in FIG. The raster lines are periodic along the X axis. The encoder ruler 105 has a grating plane corresponding to the X-Y plane, and the encoder ruler diffracts the measuring beam 112 into one or more diffraction orders in the Y-Z plane. Although the encoder ruler 105 illustrated in FIG. 1 is a periodic structure in one direction, more generally, the measurement object may include a plurality of different diffraction structures that appropriately diffract the measurement beam.

At least one of the diffraction orders of the measuring beam (labeled as beam 114) is returned to optical device 110, wherein the measuring beam combines with the reference beam to form output beam 132. For example, the primary diffracted measuring beam 114 can be a first order diffracted beam.

Output beam 132 contains phase information about the difference in optical path length between the measurement beam and the reference beam. The optical device 110 directs the output beam 132 to the detector module 130. The detector module 130 detects the output beam and transmits a signal to the electronic processor 150 in response to the detected output beam. The electronic processor 150 receives and analyzes the signal and determines information regarding one or more degrees of freedom of the measuring object 101 relative to the optical device 110.

In some embodiments, the measurement beam has a small frequency difference (e.g., a difference in the kHz to MHz range) from the reference beam to produce a desired interference signal at a frequency that approximately corresponds to the difference in frequency. This frequency is hereafter referred to interchangeably as "heterodyne" frequency or "reference" frequency. The information about the change in the relative position of the measuring object generally corresponds to the phase of the interference signal at the difference frequency. Signal processing techniques can be used to capture this phase and thus determine the relative change in distance. An example of an exemplary technique for capturing this phase and further discussion and operation of the interferometric optical encoder system can be found in U.S. Patent No. 8,300,233, the disclosure of which is incorporated herein by reference.

2 is a schematic diagram of an example encoder readhead 200 that can be used in an interferometric optical encoder system. The encoder readhead 200 includes a beam splitter 202, a reference retroreflector 204 (e.g., a cube corner reflector), and a measurement retroreflector 206 (e.g., a cube corner reflector). In other implementations, the encoder readhead 200 can include additional optical components, such as optical filters, lenses, or additional beam splitters and/or retroreflectors. The illumination source 220 directs the input beam 201 toward the beam splitter 202. The beam splitter 202 then derives the measurement beam 203 from the reference beam 205 from the input beam 201, wherein the measurement beam 203 is directed toward the target object 210 (e.g., encoder scale), diffracted, redirected by the retroreflector 206 toward the target Object 210, wherein the beam is diffracted again. The reference beam 205 travels toward the reference retroreflector 204, where the beam 205 is redirected back to the beam splitter 202. The secondary diffracted measuring beam 207 also returns to the beam splitter 202, wherein the measuring beam 207 is combined with the retroreflected reference beam 205 to form an output beam 209 which is passed to the detector 230.

However, in some implementations, separating the measurement beam from the reference beam component from the input beam 201 may be imperfect, such as partial measurement beam components not traveling through the measurement beam path and/or portions of the reference beam component. Entering the reference beam path causes an unexpected beam to "mix." Similarly, some of the retroreflected beam and the diffracted beam may travel on other undesired paths, resulting in unexpected beam mixing.

Typically, a spurious beam mixed with other beams traveling in a preferred path is referred to as a "ghost beam." Ghost beams may have different amplitudes, different phase offsets, and/or different frequencies than the beams they are combined with, resulting in a shift in the frequency or phase of the detected interfering signal, or a change in the amplitude of the detected interfering signal, Each can cause errors in the position measurement of the encoder ruler.

Low coherence optical encoder system

3 is a schematic diagram of an example beam path of optical interferometric system 300 that reduces or eliminates measurement errors associated with the presence of ghost beams. In particular, system 300 is configured to establish a defined coherence range in which ghosted beams having a beam path outside of the defined range do not interfere with the measurement beam and are thus electrically excluded by system 300.

System 300 includes a low coherent illumination source 320 that provides an input beam 301 to a coupling chamber module. The coupling chamber module includes a first interferometer chamber 306 ("heterodyne" or "modulator" chamber) coupled in series with a second interferometer chamber 308 ("test" chamber). The output from the coupling chamber module is provided to the detector 330, which in turn is coupled to the electronic processor 350. The different locations along the system are labeled as nodes (1), (2), (3), and (4). The first interferometer chamber 306 includes nodes (1) and (2). The second interferometer chamber 308 includes nodes (3) and (4).

The low coherent illumination source 320 can comprise any suitable light source that can produce Produces a beam with a low coherence. For the purposes of this disclosure, a low coherent beam is a beam having a broad spectral width (e.g., spectrally wider than a laser) or a low time coherent, such as, for example, a light emitting diode (LED) or a halogen lamp.

The temporal coherence of the Gaussian spectral shape can be expressed by the following comparison function

The function value of the comparison function C( d , λ, σ) is (normalized) contrast, d is the optical delay, σ is the Gaussian 1/e width, and λ is the spectral average wavelength. So given λ and σ, the reader can calculate this comparison, which is observed as a function of the delay (optical path difference). For example, if λ=1550 nm and σ=0.5 nm, the contrast at full width at half-maximum (FWHM) is about 1.1 mm (secondary diffraction).

The heterodyne chamber 306 includes a non-equal path chamber in which the input beam 301 is split into two distinct beams (a first segment 306a and a second segment 306b) that travel in different paths having different lengths. The difference in length between the two paths of chamber 306 defines the optical path difference (OPD) between the two beams. For example, in some implementations, the length of the first section 306a of the outer differential chamber 306 shown in FIG. 3 is longer than the length of the second section 306b of the heterodyne chamber 306, or vice versa. One or both of the chambers 306 may also include frequency shifting means 303 that give known optical frequency differences (heterodyne frequencies) or known phase change rates for the beams.

Test chamber 308 also includes a non-equal path chamber, wherein the OPD of second chamber 308 is nominally the same as the OPD of first chamber 306. That is, the first The OPD of the two chambers 308 is approximately equal to the OPD of the first chamber 306. Typically, the OPD of the first chamber (heterodyne chamber) is fixed while the second chamber (test chamber) OPD will change due to the movement of the test surface. Therefore, the OPD of the second chamber should be precisely set to ensure adequate contrast across the full range of test surface movements. Similar to heterodyne chamber 306, test chamber 308 is configured to divide an input beam into different beams that travel on different paths (measurement path 308a and reference path 308b). The length of one of the paths of the test chamber 308 can be defined in accordance with the relative position of the test object to the interferometer system (e.g., measurement path 308a), while the length of the other path of the chamber 308 (reference path 308b) is the reference path length. In some implementations, the light generated from the coupling chamber configuration interferes at a heterodyne frequency, and the phase of the interference signal is modulated proportional to the difference between the heterodyne chamber 306 and the OPD of the test chamber 308. The electronic demodulation of the heterodyne carrier can then be used to extract the basic phase change, and thus the OPD change between the two chambers. Thus, if the OPD change of the heterodyne chamber 306 is known, the OPD change of the test chamber 308, as well as the corresponding change in the position of the encoder ruler, can be determined. In addition, ghost beams having an optical path length outside the coherence length of the illumination source can be eliminated. The order in which the chambers are arranged is arbitrary. That is, the test chamber can be disposed before the heterodyne chamber or after the heterodyne chamber.

During operation of system 300, low dim light from illumination source 320 enters the heterodyne chamber at node (1). As described above, the input beam 301 is split into two different beams that travel in different paths having different path lengths x. The first path 306a in the heterodyne chamber 306 has a path length x ₀ , and the second path 306b in the heterodyne chamber 306 has a predetermined OPD x _h , so all path lengths in the second path are x ₀ +x _h . In the present example, the second path 306b of the heterodyne chamber also includes a frequency shifting device 303 (eg, an acousto-optic modulator or photo-modulator formed of quartz or TeO ₂₎ that is fed to the chamber 306. The difference in optical frequency between the lights in the two segments. Thus, the output of the heterodyne chamber 306 at node (2) contains light having a frequency ω and light moving to a second different frequency ω', where ω' = ω + ω _h , where ω _h is heterodyne frequency.

Then heterodyne light from the chamber 306 prior to entering the test chamber 308 travels at a distance x ₁ from a node (3), wherein x ₁ is the distance between the two chambers. The first path 308a in the test chamber 308 has an optical path length x ₂ and the second path 308b in the test chamber 308 has an optical path length x ₂ + x _S , where x _S is the adjustable OPD of the test chamber. For example, in some implementations, x ₂ + x _S corresponds to the length of light traveled along a reference path in test chamber 308, where x _S can be modified by modifying the position of the retroreflector on which the light is incident. Adjust it.

In the exemplary configuration of Figure 3, the path length of each of the two chambers is set such that the test chamber OPD is approximately equal to the heterodyne chamber OPD within the coherence length of the illumination source. In other words, the difference between the OPDs of the two chambers is given by | x _h -x _S |<CL. Assuming that x _h and x _S are also much larger than the coherence length, the electric field at the nodes indicated in Figure 3 is proportional (ignoring normalization):

(1): e ^iωt

(2): Where k = ω / c and k `= ω `/ c

(3):

(4): among them

At node (4), detector 330 records the squared modulus of the electric field. The expression of the squared coefficient can be obtained by assigning the unknown terms A, B, C, and D respectively to the four exponential terms of the electric field at node (4). The squared coefficient results in 16 unknowns, which can be expressed as AA*+AB*+AC*+AD*+BA*+BB*+BC*+BD*+CA*+CB*+CC*+CD*+DA *+DB*+DC*+DD*. The four generated unknown constants contain the term "self-interference" (ie, AA*, BB*, CC*, and DD*). The self-interference term corresponds to a constant (i.e., zero frequency) background signal and therefore does not constitute an interference signal. Similarly, the unknown constants AB*, BA*, CD*, DC* are also related to constant background signals and can be ignored.

The unknowns AC*, CA*, BD* and DB* are related to signals with the correct heterodyne frequency (k-k') but with an optical path length (OPL) equal to | _xh |. As mentioned above, x _{h is} much larger than the CL of the illumination source. Therefore, such a signal also constitutes a partial constant background and can be ignored.

Similarly, the terms AD* and DA* are related to signals having the correct heterodyne frequency and optical path length |x _h +x _S |. Assuming that x _h and x _S both exceed CL, the corresponding signal also constitutes a background and can be ignored.

However, the terms BC* and CB* have the correct heterodyne signal frequency and optical path length equal to | x _h -x _S |, which is very close to zero and is within the CL of the illumination source. Therefore, the signals related to BC* and CB* are the desired signals. The sum of the unknown constants BC* and CB* can be expressed as:

Where ω _h = ω - ω ` and k = ω / c , where c is the speed of light. The argument of the last term can be ignored as a small constant (eg ω _h 1MHz and x _h 10mm is 30 millirads (mrad), making

The first term of the preceding formula is the carrier term. The second, intermediate term is a small constant phase contribution from the overall fixed path length. Increasing the distance between the two chambers (x ₁ ) changes the phase of the second term, but only very slowly because it is proportional to the heterodyne frequency rather than the optical frequency of the illumination source. The separation distance x ₁ between the heterodyne chamber and the test chamber can be very large, allowing the heterodyne chamber to be remote from the test chamber. The last term of the preceding equation is the desired phase and is proportional to the OPD difference between the test chamber and the heterodyne chamber (ie, x _S -x _h ). To obtain the phase of the test chamber separately, the heterodyne chamber can be configured to have a constant or fixed OPD such that the phase change is solely due to a change in the path length of a section of the test chamber. Alternatively, the heterodyne chamber can be monitored by coupling the heterodyne chamber to the chamber of another fixed OPD.

The frequency shifting device 303 can generate heterodyne frequency differences in the two segments of the first chamber in a variety of ways. For example, the frequency shifting device 303 can include an acousto-optical modulator (AOM) device that is inserted into one or both sections of the heterodyne chamber, wherein the modulator in each segment is Drive at different frequencies. The difference between the two frequencies (either between the frequency of the single modulator in one segment and the frequency of the illumination in the other segment) corresponds to the heterodyne frequency. In another example, the frequency shifting device 303 can An electro-optic phase modulator (EOM) is included which is integrated in the first segment of the heterodyne chamber and is driven by a waveform (eg, a sawtooth waveform) having an amplitude that produces a 2π phase shift . The frequency of this waveform corresponds to the heterodyne frequency. The foregoing method is commonly referred to as a phased transponder (Serrodyne) method. Alternatively, in some implementations, two phase modulators are used, each phase of the heterodyne chamber having a phase modulator, wherein the modulators are phase modulated with an amplitude of π but with opposite phases The repeater method is driven at the same time to produce the same result. In some implementations, a phase modulation repeater method is used to generate a constant heterodyne frequency such that a simple Fourier transform can be applied to the detected interference signal to recover the phase.

Various embodiments of interferometer system 300 can be utilized. For example, FIG. 4 is a schematic diagram of an example encoder system 400 that uses an encoder read head as the test chamber 408. The input to the test chamber 408 is provided by a heterodyne chamber 406 that contains two different optical paths having different fiber lengths. In one example, the heterodyne chamber 406 uses a polarization-maintaining (PM) fiber to direct light from the low coherent illumination source 420. Alternatively, or additionally, light in the segments of chamber 406 can be guided in free space using optical components such as lenses and mirrors. The first section 411 of the heterodyne chamber 406 has an additional length relative to the second section 413 to set the chamber OPD. Further, the photo modulator 403 is integrated in the first segment 411 as a frequency shifting device. Using the phase modulation repeater method described above, the modulator 403 shifts the frequency of the light output by the heterodyne chamber 406 from that segment by the heterodyne frequency f _h . The coherence length of source 420 is modified according to the expected range of motion perpendicular to the plane of the grating. To minimize signal (interference) losses, the coherence length must be longer than the expected movement perpendicular to the grating plane. The output of the heterodyne chamber 406 is then passed to the test chamber 408.

Test chamber 408 includes a beam splitter 422, a measurement retroreflector 424, and a reference retroreflector 426 (e.g., a cube corner reflector). In some implementations, the retroreflector and/or beam splitter 422 can be fixed to an adjustable mount that allows the retroreflector and/or beam splitter to move in one or more directions. Beam splitter 422 separates the input light into a measurement path and a reference path. Light traveling along the measurement path is diffracted by the encoder ruler 405 and returned to the beam splitter 422 where the diffracted light is combined with the reference light, which has been reflected by the reference retroreflector 426. The combined light is then transmitted to photodetector 430. The processor 450 analyzes the signal received by the photodetector 430 to determine phase information.

The OPD of the test chamber 408 corresponds to the difference in optical path length between the measurement of the encoder readhead and the reference path. If the OPD difference between the heterodyne chamber 406 and the test chamber 408 is less than the source coherence length, interference occurs at the photodetector 430. The phase obtained from photodetector 430 is proportional to the difference in OPD between the heterodyne and the encoder chamber. To obtain the phase of the test chamber 408 alone, the reader can subtract the phase of the OPD corresponding to the heterodyne chamber 406. One technique for obtaining the phase of the test chamber 408 involves subtracting the phase from the fixed OPD chamber, the OPD of the fixed OPD chamber being constrained to be substantially the same as the heterodyne chamber OPD within the illumination coherence length, and ideally The ground is equal to the heterodyne chamber OPD. For example, Figure 4 shows The interferometer chamber 440 is configured to have the same OPD as the heterodyne chamber 406. Prior to recombining the light from the two paths, the fixed interferometer chamber 440 separates the light incident into the chamber 440 into two paths having a particular OPD. Photodetector 460 receives an output signal from fixed interferometer chamber 440. The processor 450 is coupled to the detector 460. The coupling of the processor to the photodetector 460 is not shown for ease of viewing. The processor 450 extracts phase information from the signal received by the photodetector 460, and subtracts the phase from the phase information obtained from the photodetector 430 to obtain the phase of the test chamber 408 alone, and thus obtains the test. The displacement information of the object. It is noted that for each interferometer in system 400, the OPD should be much larger than the source coherence length to minimize errors that may occur due to the presence of ghost beams.

In some embodiments, the encoder readhead can be configured to make the test chamber OPD adjustable. For example, Figure 5 is a schematic illustration of a test chamber 508 of an encoder system in which the test chamber includes an adjustable encoder read head. In particular, the encoder readhead includes a measurement retroreflector 524 (eg, a cube corner reflector), a quarter wave plate 525, a beam splitter 522, and an adjustable reference retroreflector 526 (eg, attached to an adjustable Cube corner reflector for the base). The beam splitter 522 includes a non-polarizing beam splitter portion 523 and a polarization beam splitter portion 521.

During operation of the encoder system, light having a suitable polarization (eg, S-polarized light) is provided from the heterodyne chamber 506 and incident on the non-polarized beam splitter portion 521 of the primary beam splitter cube 522. In some implementations, the heterodyne chamber 506 is located after the test chamber, but before the encoder ruler 505. Assuming that the encoder ruler 505 has a reflection coefficient R _G for the desired diffraction order, the beam splitter should be configured to reflect an input beam of approximately 1/(1+R _G ² ) into the test beam (the test beam is redirected towards The encoder ruler 505) transmits the remainder of the input beam to the reference beam to balance the reference and test strength.

The reference beam passes through the 1/4 wave plate 525, to the adjustable reference retroreflector 526, again through the 1/4 wave plate 525 to change the polarized light (eg, from S-polarized light to P-polarized light), through the primary beam splitter cube 522. The polarizing beam splitter portion 521 is combined with the test beam. The position of the reference retroreflector 526 can be adjusted in the X direction in this example to set the test chamber OPD to be nominally the same as the OPD of the heterodyne chamber 506. Alternatively, in some implementations, the reference retroreflector 526 can be fixed to the beam splitter cube 522, and the distance of the beam splitter cube 522 relative to the encoder ruler 505 can be adjusted.

Figure 6 is a schematic illustration of an example encoder readhead in which reference retroreflector 626 is fixed to an adjustable beam splitter cube portion 622 through a quarter wave plate 625, wherein cube 622 is structurally similar to Fig. 5 The beam splitter cube 522 is shown. In the example of Figure 6, the position of the cube 622 itself can be adjusted along the Z direction (eg, by fixing the cube 622 to the adjustable base) to set the OPD of the test chamber to be nominally the same as the heterodyne cavity. The OPD of chamber 606. In some implementations, a combination of encoder readhead configurations shown in Figures 5 and 6 can be used, wherein both the reference retroreflector and the beam splitter cube are configured to have an adjustable position (eg, using one or more Actuators, such as electromechanical actuators).

The various encoder system geometries can be modified to take advantage of the same general configuration shown in FIG. For example, in some embodiments, the configuration shown in FIG. 3 can be achieved by: 1) replacing the encoder system illumination source with a low coherent illumination source and a heterodyne chamber, and 2) coupling the heterodyne The output of the chamber to the pre-existing test chamber, and 3) ensure that the OPD of the two chambers meets the limits of unwanted unwanted ghost beams (eg | x _h -x _S |<CL and OPD is much larger than CL) . In some embodiments, the OPD requirements can be met without any substantial changes to the configuration of the test chamber. Rather, the test chamber is modified only to set the optical path length of the test path and to limit the range of variation allowed in the path.

Figure 7 is a schematic diagram showing a cross section of an example of a coding head that has been modified to operate as a combination with a low coherence source and a heterodyne chamber (such as a heterodyne chamber such as shown in Figure 4). Test chamber geometry. A description of the operation and design of the interferometer system that was previously included in the coding head can be found in U.S. Patent No. 7,440,113, the disclosure of which is incorporated herein by reference.

As shown in FIG. 7, the test chamber 708 includes a retroreflector 726 (e.g., a cube corner reflector), a beam splitter 722, first and second polarization changing elements 721a, 721b, and third and fourth polarization changing elements 723a. , 723b, and hybrid polarizer 725 (eg, a sheet polarizer or a cube polarizer). Examples of polarizing altering elements include, but are not limited to, wave plates, such as 1/4 wave plates and 1/2 wave plates. The third and fourth polarizing change elements 723a and 723b may comprise a reflective coating (eg, a reflective dielectric film stack or a metal-containing mirror coating, such as aluminum, silver, or gold) to reflect incident light back toward the beam splitting 722.

Light comprising two orthogonally polarized components is provided from the heterodyne chamber. At the interface 750 of the beam splitter 722, the input light from the heterodyne chamber is split into a measurement beam and a reference beam in accordance with the difference in polarization of the components of the input beam. For example, the measuring beam can have a first type of polarization (e.g., p-polarized light), wherein the measuring beam traverses the beam splitter interface with the first polarization changing element 721a such that it has a Littrow angle 709 (i.e., where incident The angle is equal to the angle of reflection) incident on the encoder ruler 705. The diffraction of the outgoing measuring beam traverses the first polarization changing element 721a, resulting in the beam having a second polarization type (e.g., s-polarized light). The diffracted measurement beam is reflected at the beam splitter interface 750, travels through the retroreflector 726, is again reflected at the beam splitter interface 750, and traverses the second polarization changing element 721b. The second pass-through measuring beam is incident on the encoder ruler 705 at a Littrow angle 709. The second pass of the outgoing measuring beam is in line with the incident beam and again traverses the second polarization changing element 721b to become a second passing measuring beam having a first polarization type (eg p-polarized light). The second pass of the p-polarized light passes across the beam splitter interface 750 and the hybrid polarizer 725 to the detector 730.

The reference beam formed at the interface 750 of the beam splitter has a second polarized light (e.g., s-polarized light) that is different from the polarized light of the measuring beam taken from the input beam at interface 750. The reference beam is then reflected from the third polarization changing element 723a, traveling back through the interface 750 toward the retroreflector 726, where the reference beam is redirected back through the interface 750. After passing through the interface 750 a second time, the reference beam is reflected from the fourth polarization changing element 723b and then reflected from the beam splitter interface 750 toward the detector 730. Prior to reaching detector 730, the reference beam is coupled to the measuring beam by hybrid polarizer 725.

In the example shown in FIG. 7, the length of the test path optical path (and thus the chamber OPD) can be modified by adjusting the distance traveled by the first pass and the second pass measurement beam from the beam splitter 722 to the encoder ruler 705. . For example, the configuration including beam splitter 722, retroreflector 726, and polarization changing elements can be shifted or shifted away from encoder scale 705 along path 760 toward encoder scale 705, where the path is at the angle of the Litero and the encoder The ruler 705 intersects. Alternatively, or additionally, the reference path optical path length can be modified, such as by adjusting the position of the retroreflector 726 relative to the beam splitter 722.

Figure 8A is a block diagram of another example of a coding head of a position measuring device that has been modified to operate as a test chamber in combination with a low coherence source and a heterodyne chamber. Figure 8B is a front elevational view of an embodiment of a four-grating interferometer according to the beam path shown in Figure 1. A description of the operation and design of a four-grating interferometer system that was previously included in the coding head can be found in U.S. Patent No. 7,019,842, the disclosure of which is incorporated herein by reference. The position measuring device includes a ruler and a scanning unit, and the scanning unit moves in the measuring direction with respect to the ruler. The scanning unit includes a scanning grating, a ridged corner post and a photodetector element. The ridged corner post has a ridge oriented parallel to the measurement direction, the ridged corner post acting as a retroreflector in the second direction, the second direction being aligned with the plane of the ruler and perpendicular to the measurement direction. The beam path shown in Fig. 8A is shown as an expanded view.

Test chamber 808 includes a grating interferometer for measuring between gratings mobile. In this interferometer, the measurement direction is the X direction. As in the previous example, the Y-axis extends in a direction perpendicular to the surface of the page.

For example, the grating interferometer of test chamber 808 is a four-grating (801, 803, 805, 807) transfer grating, wherein the gratings have the same grating constant or scale period. The "test" or "measurement" object contains rasters 801 and 807. Therefore, the movement of the gratings 801 and 807 is the target to be detected in this implementation. The ruler grating 801 is vertically illuminated by incident light from a heterodyne chamber 806 (e.g., a heterodyne chamber as shown in Fig. 4). In this example, the scale period of the grating 801 extends in the X direction. The light beam produced by the diffraction at the ruler grating 801 travels to the first scanning grating 803, which is disposed at a distance D from the ruler grating 801 (e.g., about 150 mm). The two beams are straightened by being diffracted at the first scanning grating 803 and travel to the second scanning grating 805. In this process, each of the two beams passes through two polarizing optical delay elements 820, 822 or 824, 826 (eg, 1/8 wave plates) attached to the scanning gratings to produce left circular polarization and right circular Shaped beam of light. Alternatively, one quarter wave plate can be used instead of two 1/8 wave plates.

At the second scanning grating 805, the beams are refracted to +/- the first diffraction order and travel to the ruler grating 807, which is disposed at a distance D from the scanning grating 805. At the ruler grating 807, the two circularly polarized beams are diffracted such that the beams overlap and travel along the same path after passing through the grating 807. A linearly polarized beam whose function is a function of the scale displacement in the measurement direction (X direction) is produced by superimposing two circularly polarized beams. The phase shift of the linearly polarized beam is a function of the displacement of the gratings 801, 807 along the X direction.

The grating 809 then splits the linearly polarized beam into three partial beams. The three polarizers 840, 842, 844 are configured to receive the three different beams, respectively, and are oriented such that the incident beams are phase shifted by about 120 relative to each other. Each of the three phase shifted beams is then incident on a different photodetector (eg, photodetector 830, 832 or 834). Each photodetector then alternately generates a detection signal corresponding to the beam thus detected. The resulting signals are also phase shifted by approximately 120° relative to each other. The generated signal is then passed to an electronic processor (eg, processor 150, 350 or 450), which can then be used to calculate the OPD of the test chamber 808 (eg, by using a known phase shift interferometric algorithm) . In this implementation, a retroreflector 802 (eg, a cube corner reflector) coupled to the adjustable base is interposed in the path of a beam of light. The position of the retroreflector can then be adjusted to modify the beam path length in a section of the test chamber 808, and the test chamber OPD is similarly adjusted such that the test chamber OPD is nominally equal to the OPD of the heterodyne chamber 806 (For example, the difference in OPD between the test and the heterodyne chamber is within the coherence length of the source).

Regarding the test chamber shown in Fig. 8B, the illumination is provided from the heterodyne chamber 10 (e.g., as shown in the heterodyne chamber 406 shown in Fig. 4). Additional details of the operation of the apparatus shown in Fig. 8B can be found in U.S. Patent No. 7,019,842, the disclosure of which is incorporated herein by reference. However, the modification to that system is that the position of at least one of the scanning grating (30), the 1/8 wave plate (40), and the ridged corner post (50) is made adjustable. For example, the grating 30, the 1/8 wave plate 40, and the ridged corner post 50 can be fixed to an adjustable base (not shown) such that the grating 30, the wave plate 40, The optical path length of the third beam path with the ridge column 50 (and therefore the OPD of the test chamber) can be varied.

Figure 9 is a schematic illustration of another example of a coding head/test chamber geometry that has been modified to operate in conjunction with a low coherence source and a heterodyne chamber. In particular, test chamber 908 includes an interferometer geometry that is configured to minimize optical path errors within the nature of raster fabrication by double diffraction. A description of the operation and design of an interferometer system that was previously incorporated in the coding head of Figure 9 can be found in U.S. Patent No. 4,979,826, the disclosure of which is incorporated herein by reference.

In Fig. 9, the light beam emitted from the heterodyne chamber (e.g., the heterodyne chamber 406 shown in Fig. 4) is split into two beams (beam (a) and beam by the beam splitter 901). (b)). The beam (a) passes through the beam splitter 901 and is reflected by the mirror 903 to face point 0 on the encoder ruler 905 at an angle of incidence θ ₁ with respect to the vertical line of the encoder ruler surface. On the other hand, the beam (b) is reflected by the beam splitter 901 and is reflected by the mirror 907 toward the retroreflector 902 (e.g., a cube corner reflector). The retroreflector 902 also redirects the beam (b) towards point 0 at an angle of incidence θ ₁ . The beam (a) is diffracted by the encoder ruler 905 into different diffraction orders (for example, a +1 diffraction beam, a 0 diffraction beam, and a -1 diffraction beam). Among those diffraction orders, the -1 diffraction beam-1(a) exits the encoder ruler 905 at an angle θ ₂ and is reflected back by the mirrors 911 and 909 to point 0 on the encoder ruler 905. The beam (b) is also diffracted by the encoder ruler 905 into different diffraction orders. Among the diffraction-order beams generated by the diffraction of the beam (b), the +1-radius beam +1(b) is emitted from the encoder ruler 905 at an angle θ ₂ and is reflected by the mirrors 909 and 911. Go back to point 0 on the encoder ruler 905. The reflective optical system comprising mirrors 909 and 911 is arranged such that the two beams -1(a) and +1(b) each travel in opposite directions on the common optical path and again enter point 0 at an angle θ ₂ .

Beam-1(a) is again diffracted into a plurality of different re-radiation steps. Among those re-circulating beams, -1 order, -1x2 (a) exits from the grating surface of the ruler 905 from point 0 on the encoder ruler 905. Similarly, beam +1(b) is again diffracted into a plurality of different re-radiation steps. Among those re-diffracted beams, +1 order, +1 x 2 (b) exits from the grating surface of the ruler 905 from point 0 on the encoder ruler 905. The beam -1x2(a) and the beam +1x2(b) exit from the common point 0 in the same direction, and their optical paths overlap each other, so that the beam -1x2(a) and +1x2(b) interfere with each other and are optically detected. The detector 913 provides an interference light signal when detected. The beam -1x2(a) corresponds to a beam that has been subjected to the -1 order diffraction twice of the encoder ruler 905. The phase of the beam -1x2(a) is thus delayed by φ _a for each relative movement amount x of the encoder ruler 905 in either direction of the arrow 920. Similarly, the phase of the beam +1x2(b) is premature φ _b for each relative movement x of the diffraction ruler 905 in either direction of the arrow 920. The interference signal generated by the interference of the two beams at the photodetector 913 is transmitted to an electronic processor (such as, for example, an electronic processor 150, 350, or 450), and the electronic processor can capture the phase of the interference signal. By using the output from the heterodyne chamber and incorporating mirror 907 and retroreflector 902, the optical path length of one of the two beams can be varied to produce a nominal match within the coherence length of the illumination source. The chamber OPD of the differential chamber OPD.

The beam path configuration shown in FIG. 3 above can also be applied to a distance measuring interferometer, such as, for example, a plane mirror interferometer (PMI), a high stability PMI, and a differential PMI. For example, Figure 10 is a schematic diagram showing an example cross-sectional view of a multi-channel distance measuring interferometer having a common reference path that has been modified to operate in conjunction with a low coherent source and a heterodyne chamber. A description of the operation and design of the multi-channel distance measuring interferometer prior to the modification shown in Fig. 10 can be found in U.S. Patent No. 7,224,466, the entire disclosure of which is incorporated herein by reference.

System 1008 includes a beam splitter 1001 whose position relative to the measurement reflector 1003 on the test object can be modified. In other words, the test chamber corresponds to the area between the measurement reflector 1003 (eg, a mirror) and the quarter wave plate 1005, wherein the distance between the reflector 1003 and the wave plate 1005 is adjustable. Thus, the optical path length of the beam traveling in system 1008 can be varied such that the OPD of test chamber 1008 is nominally the same as the OPD of heterodyne chamber 1006.

In addition to the beam splitter 1001, the reflector 1003, and the quarter-wave plate 1005, the system 1008 also includes a quarter-wave plate 1007, a reference reflector 1009 (eg, a mirror), and retroreflectors 1011 and 1013 ( For example, a cube corner reflector) separates the optical element 1015 from the beam. The heterodyne output from heterodyne chamber 1006 corresponds to input beam IN, which contains two components (imaginary and solid lines) with orthogonal linear polarization. Although the reference reflector 1009 is shown fixed to the quarter-wave plate 1007 in FIG. 10, and thus is also fixed to the beam splitter 1001, the reflector 1009 can also be individually provided. Placed on, for example, an adjustable base.

The polarized beam splitter 1001 separates the equal components of the input beam IN according to linear polarization to produce a common measurement beam and a common reference beam. The measurement beam and the reference beam are referred to as "shared" because the configuration shown in Figure 10 is used to create two different output channels, which can also measure skew. The common measuring beam is a polarized component of the input beam IN that the polarizing beam splitter 1001 initially transmits toward the quarter-wave plate 1005, and the common reference beam is the input beam IN. The polarizing beam splitter 1001 initially faces the quarter. A polarized component reflected by a wave plate 1007. The shared measuring beam travels through the path MS through the quarter-wave plate 1005 to the measuring mirror 1003, from the measuring mirror 1003, and travels through the path MS' back through the quarter-wave plate 1005 into the polarizing beam splitter 1001. The common measuring beam is incident perpendicular to the measuring mirror 1003, and the paths MS and MS' sharing the measuring beam are in the same straight line.

Passing the shared measuring beam twice through the quarter-wave plate 1005 has the effect of rotating the polarized light of the common measuring beam by 90°, resulting in the common measuring beam being reflected from the beam splitting interface 1050 in the polarizing beam splitter 1001 toward the beam splitting optical element. 1015. The shared measuring beam thus passes through the polarizing beam splitter 1001 and enters the beam splitting optical element 1015.

The polarized beam splitter 1001 also reflects a component of the input beam IN at the interface 1050 to produce a common reference beam that travels along the path RS through the quarter-wave plate 1007 to the reference mirror 1009. The common reference beam is reflected back along the path RS', passes through the quarter-wave plate 1007 and returns to the polarized beam splitter 1001. After sharing the reference beam The polarized beam splitter 1001 transmits linearly polarized light, and the common reference beam passes through the polarized beam splitter 1001 into the beam splitting optical element 1015 (substantially in line with the common measuring beam).

The beam splitting optical element 1015 splits the common measuring beam and the common reference beam into individual beams corresponding to the measuring axis. Because there is a non-polarized coating at the beam splitting interface 1060 on the beam splitter 1015, half of the energy of the shared measuring beam and half of the energy of the common reference beam are thus coated by the beam splitter and enters the back associated with the first measuring axis. The emitter 1011. The other half of the shared measurement and common reference beam is coated from the beam splitter and then enters the retroreflector 1013 associated with the second measurement axis.

The retroreflector 1011 reflects and offsets the individual beams corresponding to the first measurement axis. The first individual beam returns to the polarized beam splitter 1001, and the polarized beam splitter 1001 splits the first individual beam at the interface 1050 into a first measuring beam and a first reference beam associated with the first measuring axis. The first measuring beam is reflected from the polarizing beam splitter interface 1050 in the polarized beam splitter 1001 and travels through the quarter wave plate 1005 to the measuring reflector 1003 along the path M1. The first measuring beam is then reflected from the measuring mirror 1003 and returned to the polarizing beam splitter 1001 along the path M1'.

The reflection of the first measuring beam from the measuring mirror 1003 causes the same but opposite angular error which counteracts the change between the first measuring and the reference beam. After traversing the paths R1 and R1' to and from the reference mirror 1009 and from the beam splitter interface 1050 in the polarized beam splitter 1001, the first reference beam is thus parallel to the first measurement path M1', and the first measurement Formed with the reference beam to form a first measurement axis The output beam OUT1, wherein the output beam OUT1 is detected by the first detector 1040 (eg, a photodetector).

The second individual beam is reflected from the retroreflector 1013 and enters the polarized beam splitter 1001, wherein the polarized beam splitter 1001 splits the second individual beam into a second measuring beam and a second reference beam. Before the second measurement and the reference beam exit to form the second output beam OUT2 corresponding to the second measurement axis, the second measurement beam travels to the paths M2 and M2' to measure the reflector 1003 back and forth, and the second reference beam travels in the path R2 and R2' go back and forth to the reference reflector 1009, wherein the output beam OUT2 is detected by the second detector 1042 (eg, a photodetector).

The measuring device 1030 (for example, an electronic processor) is coupled to the detector 1040 and receives an output signal generated by the detector 1040 when detecting the output beam OUT1. The measuring electronics 1030 measures the first measuring beam and the first reference beam. The difference in frequency between and the calculations are reflected from the measurement mirror 1003 resulting in any Doppler shift in the first measurement beam. The measured Doppler shift includes a component caused by the reflection of the common measuring beam (i.e., the reflection from the path MS to the path MS') and the reflection of the first measuring beam (i.e., from the path M1 to the path) A component caused by the reflection of M1'). The measuring electronics 1030 thus effectively measures the moving average of the measuring mirror 1003 at two points, which should be equal to the movement of the intermediate point between the two reflections on the measuring mirror 1003.

The measuring electronics 1032 (for example, an electronic processor) is coupled to the detector 1042 and receives an output signal generated by the detector 1042 when detecting the output beam OUT2, and the measuring electronics 1032 measures the second measuring beam and the second reference. The difference in frequency between the beams is measured to reflect any Doppler shift in the second measuring beam as reflected from the measuring mirror 1003. This measured Doppler shift includes the reflection of the common measurement beam (ie, the reflection from path MS to path MS') and the reflection of the second measurement beam (ie, from path M2 to path) A component caused by the reflection of M2'). The measuring electronics 1032 thus effectively measures the moving average of the measuring mirror 1003 at two points, which should be equal to the movement of the intermediate point between the two reflections from the measuring mirror 1003.

Generally, any of the above analysis methods (including determining the phase information from the detected interference signal and the degree of freedom of the encoder ruler) can be implemented by a combination of computer hardware or software or a combination of both. For example, in some embodiments, electronic processors 150, 350, 450, 1030, and/or 1032 can be installed in a computer and coupled to one or more encoder systems, and configured to perform analysis from an encoder system. signal. The analysis can be performed using a computer program using standard programming techniques using the methods described herein. The code is applied to input data (eg, interference phase information) to perform the functions described herein and to generate output information (eg, degree of freedom information). The output information is applied to one or more output devices, such as a display. Each program can be implemented in a high-level program or object-oriented programming language to communicate with a computer system. However, the program can be implemented in a combination or machine language, if desired. In any instance, the language can be a compiled or interpreted language. In addition, the program can operate on a dedicated integrated circuit that is pre-programmed for this purpose.

Each such computer program is preferably stored on a storage medium or device (such as a ROM or a disk) that can be read by a general or special purpose programmable computer. Used to set up and operate a computer when the storage medium or device is read by a computer to perform the programs described herein. The computer program can also be saved in the cache or main memory during program execution. The analysis method can also be implemented as a computer readable storage medium configured in a computer program, wherein the storage medium is configured such that the computer operates in a particular and predetermined manner to perform the functions described herein.

Lithography tool application

The lithography tool is particularly useful in lithographic applications used to make large integrated circuits, such as computer chips and the like. Photolithography is a key technology driver for the semiconductor manufacturing industry. Overlay improvement is one of the five most difficult challenges (design rules) to fall below the 22 nm linewidth and 22 nm linewidth, see for example the International Technology Roadmap for Semiconductors, pp. 58-59 (2009). ).

Coverage is directly dependent on the performance (ie, accuracy and precision) of the measurement system used to position the wafer and the reticle (or mask) platform. Because lithography tools can produce between $50 million and $100 million per year, the economic value from improved measurement systems is significant. Every 1% increase in yield of lithography tools results in an economic benefit of approximately $1 million per year for integrated circuit manufacturers and a significant competitive advantage for lithography tool suppliers.

The function of the lithography tool is to direct spatially patterned radiation onto the photoresist-coated wafer. The procedure involves determining where in the wafer the radiation is to be received (aligned) and applying the photoresist (exposure) radiated to that location.

During exposure, the radiation source illuminates the patterned reticle, which scatters the radiant Shooting produces spatially patterned radiation. Photomasks are also known as masks, and these terms are used interchangeably below. In the case of reduction lithography, the reduced magnification lens collects the scattered radiation and forms a reduced image of the reticle pattern. Alternatively, in the case of proximity printing, the scattered radiation travels a small distance (typically a micron size) before contacting the wafer to produce a 1:1 image of the reticle pattern. The radiation initiates a photochemical process in the photoresist that converts the radiation pattern into a latent image within the photoresist.

In order to properly position the wafer, the wafer contains alignment marks on the wafer that can be measured by a dedicated sensor. The measured position of the alignment mark defines the location of the wafer within the tool. This information, along with the desired patterning of the wafer surface, directs the alignment of the wafer relative to the spatially patterned radiation. Based on this information, a transferable platform supporting a photoresist-coated wafer moves the wafer such that the radiation will expose the correct location of the wafer. In some lithography tools, such as lithography scanners, the mask is also positioned on a transferable platform that moves in unison with the wafer during exposure.

Encoder systems (such as those previously discussed) are important components of the positioning mechanism that controls the position of the wafer and the reticle, and temporarily store the reticle image on the wafer. If such an encoder system includes the above features, the accuracy of the distance measured by the system can be increased and/or maintained for a longer period of time without downline maintenance, resulting in higher yields due to increased yield and less tool downtime.

Typically, lithography tools (also known as exposure systems) typically include an illumination system and a wafer positioning system. The illumination system includes a source of radiation for providing radiation (eg, ultraviolet light, visible light, x-rays, electrons, or ionizing radiation), and a reticle or mask for distributing the pattern to the radiation, thereby creating a spatially patterned radiation. Moreover, for the case of reducing magnification lithography, the illumination system can include a lens combination for imaging spatially patterned radiation onto the wafer. The imaged radiation exposes the photoresist coated on the wafer. The illumination system also includes a masking platform for supporting the mask, and a positioning system for adjusting the position of the masking platform relative to the radiation being directed through the mask. The wafer positioning system includes a wafer platform for supporting the wafer and a positioning system for adjusting the position of the wafer platform relative to the imaged radiation. The fabrication of the integrated circuit can include multiple exposure steps. For a general reference to lithography, see, for example, JR Heats and BWSmith, in Microlithography: Science and Technology (Marcel Dekker, Inc., New York, 1998), the contents of which are hereby incorporated by reference.

The encoder system described above can be used to accurately measure the position of each of the wafer platform and the mask platform relative to other components of the exposure system, such as a lens assembly, a radiation source, or a support structure. In such an example, the optical device of the encoder system can be attached to a fixed structure, and the encoder ruler can be attached to a movable element (eg, one of a mask and a wafer platform). Alternatively, the condition can be reversed, the optical device attached to the movable item and the encoder ruler attached to the fixed item.

More generally, such an encoder system can be used to measure the position of any component of the exposure system relative to any other component of the exposure system, wherein the optical device can be attached to or supported by a component, and the encoder ruler can Attached to or supported by another component.

An example of a lithography tool 1800 using an interferometric system 1826 is shown in FIG. The encoder system can be used to accurately measure the position of wafers (not shown) within the exposure system. Here, platform 1822 is used to position and support the wafer relative to the exposure station. The scanner 1800 includes a frame 1802 that carries other support structures and various components carried on those structures. The exposure pedestal 1804 has a lens housing 1806 mounted on top of it, and a reticle or mask platform 1816 is mounted on top of the lens housing 1806 for supporting the reticle or mask. A positioning system for positioning the mask relative to the exposure station is schematically indicated by element 1817. Positioning system 1817 can include, for example, piezoelectric transducer elements and corresponding control electronics. Although not included in this described embodiment, one or more of the encoder systems described above can also be used to accurately measure the position of the mask platform and other movable components, their position must be in the process of fabricating the lithographic structure. It is accurately monitored (see supra Sheats and Smith Microlithography: Science and Technology ).

Hanged beneath the exposure pedestal 1804 is a support pedestal 1813 that carries the wafer platform 1822. The platform 1822 includes a measurement object 1828 for diffracting a measurement beam 1854 that is directed by the optical device 1826 to the platform. A positioning system for positioning the platform 1822 relative to the optical device 1826 is schematically indicated by element 1819. Positioning system 1819 can include, for example, piezoelectric transducer elements and corresponding control electronics. The measuring object is diffracted and the measuring beam is reflected back to the optical device, and the optical device is mounted on the exposure base 1804. The encoder system can be any of the foregoing embodiments.

During operation, a radiation beam 1810 (e.g., an ultraviolet beam from an ultraviolet (UV) laser (not shown)) passes through beam shaping optics 1812 and, after being reflected from mirror 1814, travels downward. After that, the radiation beam passes A mask (not shown) carried by the mask platform 1816 is passed through. A mask (not shown) is imaged onto a wafer (not shown) on the wafer platform 1822 through a lens assembly 1808 carried in the lens housing 1806. The base 1804 and the various components it supports are isolated from ambient vibrations by a damping system depicted by springs 1820.

In some embodiments, one or more of the aforementioned encoder systems can be used to measure displacement along multiple axes and such as, but not limited to, angles associated with wafers and reticle (or mask) platforms. In addition, in addition to the UV laser beam, other beams can be used to expose the wafer, such as x-ray beams, electron beams, ion beams, and visible beams.

In some embodiments, optical device 1826 can be positioned to measure the change in position of the reticle (or mask) platform 1816 or other movable components of the scanner system. Finally, in addition to or not being a scanner, the encoder system can be used in a similar manner with lithography systems that include steppers.

As is well known in the art, lithography is a critical part of the manufacturing process used to fabricate semiconductor devices. For example, U.S. Patent No. 5,483,343 outlines the steps of such a manufacturing process. These steps are described below with reference to Figures 12A and 12B. Fig. 12A is a flow chart of a procedure for manufacturing a semiconductor device, such as a semiconductor wafer (e.g., IC or LSI), a liquid crystal panel, or a CCD. Step 1951 is a design procedure for designing a circuit of a semiconductor device. Step 1952 is a program for fabricating a mask based on the circuit pattern design. Step 1953 is a program for fabricating a wafer by using a material such as germanium.

Step 1954 is a wafer process, referred to as a pre-program, in which the circuit is formed on the wafer by lithography by using the mask and wafer thus prepared. In order to form a circuit on the wafer that corresponds to the sufficient spatial resolution of those patterns on the mask, interferometric positioning of the lithography tool relative to the wafer is desirable. The interferometric methods and systems described herein are particularly useful to improve the utility of lithography used in wafer programs.

Step 1955 is a combined procedure, referred to as a subsequent procedure, in which the wafer processed in step 1954 is formed as a semiconductor wafer. This step consists of a combination (cutting and wiring) and a package (wafer sealing). Step 1956 is an inspection procedure in which the operability check, durability check, etc. of the semiconductor device produced in step 1955 are performed. With these procedures, the semiconductor devices are completed and shipped (step 1957).

Figure 12B is a flow chart showing the details of the wafer program. Step 1961 is an oxidation process for oxidizing the surface of the wafer. Step 1962 is a CVD process for forming an insulating film on the surface of the wafer. Step 1963 is an electrode forming process for forming an electrode on the wafer by vapor deposition. Step 1964 is an ion implantation procedure for implanting ions into the wafer. Step 1965 is a photoresist program for applying a photoresist (photosensitive material) to the wafer. Step 1966 is an exposure process for printing a circuit pattern of the mask onto the wafer by exposure (e.g., lithography) and through the exposure apparatus described above. Again, as described above, the use of the interferometric systems and methods described herein will improve the accuracy and resolution of such lithography steps.

Step 1967 is a development process for developing the exposed wafer. Step 1968 is an etch process for removing portions of the photoresist image that are not being developed. Step 1969 is a photoresist separation process for separating the photoresist material remaining on the wafer after the etching process is performed. By repeating these procedures The circuit pattern can be formed and superimposed on the wafer.

The above encoder system can also be used in other applications where the relative position of an object needs to be accurately measured. For example, in applications where a writing beam (eg, laser, x-ray, ion, or electron beam) marks a pattern on a substrate as the substrate or beam moves, an encoder system can be used to measure the substrate to the writing beam. Relative movement.

Many embodiments have been described. However, it will be appreciated that various modifications may be made without departing from the scope and spirit of the invention. Other embodiments are within the scope of the following patent claims.

0‧‧‧ points

10‧‧‧heterodyne chamber

30‧‧‧Raster

40‧‧‧1/8 wave plate

50‧‧‧ ridged column

100‧‧‧Interferometric encoder system

101‧‧‧Measurement objects

105‧‧‧Encoder ruler

110‧‧‧Optical device

112, 114‧‧‧Measurement beam

120‧‧‧Light source module

122‧‧‧Input beam

130‧‧‧Detector Module

132‧‧‧Output beam

150‧‧‧Electronic processor

200‧‧‧Encoder read head

201‧‧‧Input beam

202‧‧‧beam splitter

203‧‧‧Measurement beam

204‧‧‧Reference retroreflector

205‧‧‧Reference beam

206‧‧‧Measurement retroreflector

207‧‧‧Secondary diffraction measuring beam

209‧‧‧Output beam

210‧‧‧Target object

220‧‧‧Environment source

230‧‧‧Detector

300‧‧‧Interferential system

301‧‧‧Input beam

303‧‧‧ Frequency mobile device

306‧‧‧First interferometer (heterodyne) chamber

306a‧‧‧ first paragraph

306b‧‧‧second paragraph

308‧‧‧Second interferometer (test) chamber

308a‧‧‧Measurement path

308b‧‧‧Reference path

320‧‧‧Low coherent illumination source

330‧‧‧Detector

350‧‧‧Electronic processor

400‧‧‧Encoder system

403‧‧‧Photoelectric transducer

405‧‧‧Encoder ruler

406‧‧‧heterodyne chamber

408‧‧‧Test chamber

411‧‧‧ first paragraph

413‧‧‧ second paragraph

422‧‧‧beam splitter

424‧‧‧Measurement retroreflector

426‧‧‧Reference retroreflector

430‧‧‧Photodetector

440‧‧‧Fixed interferometer chamber

450‧‧‧Processor

460‧‧‧Photodetector

505‧‧‧Encoder ruler

506‧‧‧heterodyne chamber

508‧‧‧Test chamber

521‧‧‧Polarized beam splitter

522‧‧‧beam splitter

523‧‧‧Non-polarized beam splitter

524‧‧‧Measurement retroreflector

525‧‧‧1/4 wave plate

526‧‧‧Adjustable reference retroreflector

606‧‧‧heterodyne chamber

622‧‧‧beam splitter cube

625‧‧‧1/4 wave plate

626‧‧‧Reference retroreflector

708‧‧‧Test chamber

709‧‧‧ Litro angle

721a‧‧‧First polarizing change element

721b‧‧‧Second polarizing change element

722‧‧‧beam splitter

723a‧‧‧ Third polarizing change element

723b‧‧‧fourth polarization changing component

725‧‧‧Hybrid polarizer

726‧‧‧Rejector

730‧‧‧Detector

750‧‧" interface

801, 803, 805, 807‧‧ ‧ grating

806‧‧‧heterodyne chamber

808‧‧‧Test chamber

820, 822, 824, 826‧‧‧ polarized optical delay elements

830, 832, 834‧‧ ‧ optical detector

840, 842, 844‧‧ ‧ polarizers

901‧‧‧beam splitter

902‧‧‧Rejector

903‧‧‧Mirror

905‧‧‧Encoder ruler

907‧‧‧Mirror

908‧‧‧Test chamber

909, 911‧‧‧ mirror

913‧‧‧Photodetector

920‧‧‧ arrow

1001‧‧‧beam splitter

1003‧‧‧Measurement reflector (mirror)

1005‧‧‧ quarter wave plate

1006‧‧‧heterodyne chamber

1007‧‧‧ Quarter Wave Plate

1008‧‧‧ system

1009‧‧‧Reference reflector

1011, 1013‧‧‧ retroreflector

1015‧‧‧ Beam splitting optics

1030, 1032‧‧‧Measuring electronics

1040‧‧‧First detector

1042‧‧‧Second detector

1050‧‧‧beam separation interface

1060‧‧‧beam separation interface

1800‧‧‧ lithography tool (scanner)

1802‧‧‧Frame

1804‧‧‧ exposure base

1806‧‧‧ lens housing

1808‧‧‧ lens combination

1810‧‧‧radiation beam

1812‧‧‧beam shaping optics

1813‧‧‧Support base

1814‧‧‧Mirror

1816‧‧‧Photomask or mask platform

1817‧‧‧ Positioning System

1819‧‧‧ Positioning System

1820‧‧ spring

1822‧‧‧ platform

1826‧‧‧Interferometric system (optical device)

1828‧‧‧Measurement objects

1854‧‧‧Measurement beam

D‧‧‧Distance

IN‧‧‧Input beam

M1, M1’‧‧‧ Path

M2, M2’‧‧‧ Path

MS, MS’‧‧‧ Path

OUT1‧‧‧ output beam

OUT2‧‧‧second output beam

R1, R1’‧‧‧ path

R2, R2’‧‧‧ path

RS, RS’‧‧‧ Path

x ₀ ‧‧‧path length

x ₁ ‧‧‧distance

x ₂ ‧‧‧ optical path length

x _h ‧‧‧OPD

x _S ‧‧‧ Adjustable OPD for test chamber

ω, ω’‧‧‧ frequency

ω _h ‧‧‧heterodyne frequency

θ ₁ ‧‧‧ incident angle

θ ₂ ‧‧‧ angle

(1), (2), (3), (4) ‧ ‧ nodes

(a), (b) ‧ ‧ beam

-1(a)‧‧‧ Beam

+1(b)‧‧‧beam

-1, -1x2‧‧‧ re-beam

+1, +1x2(b)‧‧‧ re-diffracted beam

Figure 1 is a schematic diagram of an example of an interferometric optical encoder system.

Figure 2 is a schematic diagram of an example of an encoder readhead.

Figure 3 is a schematic illustration of an example of a beam path for an optical interferometric system.

Figure 4 is a schematic illustration of an example of an interferometric optical encoder system.

Figure 5 is a schematic illustration of an example of a test chamber.

Figure 6 is a schematic diagram of an example of an encoder read head.

Figure 7 is a schematic diagram showing an example of a partial interferometer modified to operate with a low coherent source and a heterodyne chamber.

Figure 8A is a block diagram of a partial interferometer modified to operate with a low coherent source and a heterodyne chamber.

Figure 8B is a schematic diagram showing an example of a partial interferometer modified to operate with a low coherent source and a heterodyne chamber.

Figure 9 is a schematic diagram showing an example of a partial interferometer modified to operate with a low coherent source and a heterodyne chamber.

Figure 10 is a schematic diagram showing an example of a portion of a multi-channel distance measuring interferometer modified to operate with a low coherent source and a heterodyne chamber.

Figure 11 is a schematic illustration of an embodiment of a lithography tool that includes an interferometer.

12A and 12B are flowcharts depicting a procedure for manufacturing an integrated circuit.

100‧‧‧Interferometric encoder system

101‧‧‧Measurement objects

105‧‧‧Encoder ruler

110‧‧‧Optical device

112, 114‧‧‧Measurement beam

120‧‧‧Light source module

122‧‧‧Input beam

130‧‧‧Detector Module

132‧‧‧Output beam

150‧‧‧Electronic processor

Claims (23)

A method of determining positional change information of an encoder ruler, comprising: dividing a low coherent beam into a first beam traveling along a first path of the first interferometer chamber in a first interferometer chamber And a second beam traveling along a second path of the first interferometer chamber; combining the first beam with the second beam to form a first output beam; in a second interferometer chamber Dividing the first output beam into a measurement beam traveling along one of the measurement paths of the second interferometer chamber and a reference beam traveling along a reference path of the second interferometer chamber; adjusting and the second An optical path difference (OPD) associated with the interferometer chamber; setting the OPD associated with the second interferometer chamber to be approximately equal to an OPD associated with the first interferometer chamber; The reference beam combines to form a second output beam; an interference signal is detected based on the second output beam; and the information about the change in position of the encoder ruler is determined based on phase information from the interference signal. A method for determining positional change information of an encoder ruler according to claim 1 of the scope of the patent application, wherein between the OPD associated with the second interferometer chamber and the OPD associated with the first interferometer chamber A difference is less than or equal to the coherence length of the low coherent beam. According to the decision of the first paragraph of the patent application, the position of the encoder ruler A method of varying information, wherein adjusting the OPD associated with the second interferometer chamber comprises adjusting an optical path length (OPL) of at least one of the measurement path or the reference path. A method for determining positional change information of an encoder ruler according to the second aspect of the patent application, wherein each of the OPD associated with the first chamber and the OPD associated with the second chamber is greater than the The coherence length of a low coherent beam. The method for determining position change information of an encoder ruler according to the first aspect of the patent application, wherein the OPD of the first chamber is equal to one optical path length (OPL) of the first path and the second path A difference between an OPL, the OPL of the second path being different from the OPL of the first path. The method for determining the position change information of the encoder ruler according to the first aspect of the patent application scope, further comprising guiding the measuring beam toward the encoder ruler before combining the measuring beam with the reference beam, wherein the measuring The beam is diffracted at least once from the encoder ruler. The method of determining the position change information of an encoder ruler according to the first aspect of the patent application, further comprising a frequency of moving at least one of the first beam or the second beam in the first interferometer chamber. A method for determining position change information of an encoder ruler according to claim 7 of the scope of the patent application, wherein the second output beam comprises a heterodyne frequency, and at least one of the first beam or the second beam is moved After the frequency, the heterodyne frequency is equal to a difference between the frequency of the first beam and the frequency of the second beam. An interferometric system comprising: a low coherent illumination source; a first interferometer chamber coupled to the low coherent illumination source to receive an output of the illumination source, the first interferometer chamber being associated with a first An optical path difference (OPD); and a second interferometer chamber coupled to the first interferometer chamber to receive an output of the first interferometer chamber, the second interferometer chamber being associated with a second OPD, wherein at least one of the first OPD or the second OPD is adjustable such that a difference between the first OPD and the second OPD is less than one of the outputs of one of the low coherent illumination sources Length (CL). The interferometric system of claim 9, wherein the first OPD is fixed. The interferometric system of claim 9, wherein the second OPD is adjustable. The interferometric system of claim 9, wherein a difference between the first OPD and the second OPD is less than the coherence length (CL) of one of the low coherent illumination sources. The interferometric system of claim 9, wherein each of the first OPD and the second OPD is greater than the coherence length (CL) of the output of the illumination source. The interferometric system of claim 9, wherein the first OPD is approximately equal to the second OPD. The interferometric system of claim 9, wherein the first chamber comprises a first segment having a first optical path length (OPL) And a second segment having a second different OPL, the OPD of the first chamber being equal to the difference between the first OPL and the second OPL. The interferometric system of claim 9 wherein the first chamber comprises a frequency shifting device in the first segment, the frequency shifting device being configured to move during the operation of the interferometric system a frequency of light in a segment. The interferometric system of claim 16 wherein the frequency shifting device comprises an acousto-optic modulator or a photo-phase modulator. The interferometric system of claim 9, wherein the second chamber comprises a first segment having a first optical path length (OPL) and a second segment having a second OPL, the second The OPD of the chamber is equal to a difference between the first OPL and the second OPL. The interferometric system of claim 18, wherein at least one of the first OPL and the second OPL is adjustable. The interferometric system of claim 18, wherein the first segment corresponds to a measurement path and the second segment corresponds to a reference path. The interferometric system of claim 18, wherein the second chamber comprises a diffractive encoder ruler, each of the first OPL and the second OPL being associated with the encoder ruler A position to define. The interferometric system of claim 9 further comprising a photodetector and an electronic processor configured to be detected from the photodetector during operation of the interferometric system A signal obtains heterodyne phase information. The interferometric system of claim 22, wherein the second chamber comprises a diffractive encoder ruler, and wherein the electronic processor is configured to operate according to the heterodyne during operation of the interferometric system Phase information to obtain position information about one degree of freedom of the encoder ruler.